Recycling Perovskite Solar Cells

Recycling Perovskite Solar Cells

By Judy Tsao, Member-at-Large for the GCI

Solar energy is arguably the most abundant and environmentally friendly source of energy that we have access to. In fact, crystalline silicon solar cells have been employed in parts of the world at a comparable cost to the price of electricity derived from fossil fuels.1 The large-scale employment of solar cells, however, remains challenging as the efficiency of existing solar cells still needs to be improved significantly.

An important recent breakthrough the field of solar cells is the use of perovskite solar cells (PSC), which includes a perovskite-structured compound as the light-harvesting layer in the device (Figure 1). Perovskite is a name given to describe the specific 3-D arrangement of atoms in such materials. Even though the first PSC was reported only in 2009, its power conversion efficiency (PCE) has already been reported to exceed 20%, a milestone in the development of any new solar cells which typically takes decades of optimization to achieve.2

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Figure 1. Thin-film perovskite solar cell manufactured by vapour deposition (photo credit: Boshu Zhang, Wong Choon, Lim Glenn & Mingzhen Liu)

PSC has several advantages compared with traditional solar cells, including low weight, flexibility, and low cost.3 There are, however, several challenges that must be overcome before PSC can be brought to the market. The most common PSC to date includes CH3NH3PbI3 and related materials, which contain soluble lead (II) salts that are toxic and strictly regulated.

Interestingly, there has been a consensus in the literature that the lead content in the perovskite layer is not actually the main issue in the environmental impact of PSC production.4 Part of the reason for this conclusion is simply that the thickness of perovskite layer required would amount to less than 1000 mg of lead in one square meter of material. This value is only modest compared to lead pollution from other human sources such as lead paints or lead batteries.5

The main environmental concerns regarding PSCs appear to lie in the use of gold and high temperature processes during the manufacturing of the devices.6 It has thus been suggested that, in order to reduce the environmental impact of PSCs, recycling of raw materials is very important. In a recent study by Kadro et al., 7 a facile protocol for the recycling of perovskite solar cell was developed. The entire procedure takes place at room temperature and takes less than 10 minutes (Figure 2).

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Figure 2. Schematic process for recycling PSC components [7].

As it turns out, components of a fully assembled PSC can be extracted by sequentially placing the device in different solvents. Step 1 of the procedure uses chlorobenzene to remove the gold layer, while step two uses ethanol to dissolve CH3NH3I. This then leaves PbI2 to be the only component remaining on the device, which can be removed by just a few drops of N,N-dimethylformamide. It is also worth noting that the recycled materials can be fabricated into a complete PSC again without significant drop in performance.

Even though the discovery of PSC has only been made less than a decade ago, its potential in applications in photovoltaics has been underlined by numerous studies. It is especially gratifying to see that the environmental impacts of such devices are already under active research before PSCs are introduced to the market. While these studies have demonstrated that PSCs have low environmental impacts when properly recycled, there are other challenges still facing researchers in this field. In particular, the short lifetime of such devices needs to be improved to match that of traditional silicon-based solar cells. Nevertheless, the facile method of recycling PSCs without compromising the performance will certainly make them even more competitive than traditional solar cells.

References:

  1. Branker, K. et al. Renewable Sustainable Energy Rev. 2011, 15, 4470.
  2. Yang, W. S. et al. Science, 2015, 348, 1234.
  3. Snaith, H. J. Phys. Chem. Lett. 2013, 4, 3623.
  4. Serrano-Lujan, L. et al. Energy Mater. 2015, 5, 150119.
  5. Dabini, D. Phys., 2015, 6, 3546.
  6. Espinosa, N. et al. Adv. Energy Mater. 2015, 5, 1.
  7. Kadro, J. M. et al. Energy, Environ. Sci. 2016, 9, 3172.

 

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Green Chemistry Principle #7: Use of Renewable Feedstocks

By Trevor Janes, Member-at-Large for the GCI

7. A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.

In Video #7, Yuchan and Ian help us understand what a raw material or feedstock is, and why we need to choose feedstocks which are renewable.

They use CO2 as an example of a feedstock which plants convert into sugar via photosynthesis. We humans use this sugar as our own feedstock for many different delicious things, including cookies! Yuchan and Ian explain that for a feedstock to be renewable, it must be able to be replenished on a human timescale, whereas depleting feedstocks take much longer to be replenished, and are being used up at a faster rate by human activity.

Many common feedstocks are depleting, such as petroleum and natural gas. The petrochemical industry uses petroleum and natural gas as feedstocks to make intermediates, which are later converted to final products that people use, such as plastics, paints, pharmaceuticals, and many others.

An example of a renewable feedstock is biomass, which refers to any material derived from living organisms, usually plants. In contrast to depleting feedstocks like petroleum, we can much more easily grow new plants once we use them up, and maintain a continuous supply. If we can use bio-based chemicals to do the same tasks that we currently accomplish using petrochemicals, we move closer to the goal of having a steady, reliable supply of resources for the future.

Existing chemical technology has developed based on using readily available petroleum as feedstock to make a majority of chemicals and end products. However, the chemical technology that enables conversion from biomass into bio-based chemicals into final products people use is not yet as well developed.1 Chemical scientists with various specializations are currently involved in improving our ability to use biomass.2, 3

So, how can we implement the principle of renewable feedstocks on a day-to-day basis? Yuchan and Ian illustrate principle 7 through their choice of solvent. As we explore in the video for principle #5, we choose a solvent for a particular purpose based on properties such as boiling point, polarity, and overall impact on health and the environment. One more aspect to consider is that we can choose to use a solvent based on is its renewability. Tetrahydrofuran (THF) is a useful ether solvent, but it is synthesized industrially from petrochemicals (see below for synthesis), so it isn’t renewable. A close relative of THF is 2-methyl THF. Its structure and properties are very similar to those of THF, but the difference is that 2-methyl THF can be synthesized from bio-based chemicals which are made from renewable feedstocks. So when we substitute 2-methyl THF in for THF, we are putting principle 7 into action.

Synthesis of THF4 vs. synthesis of 2-methyl THF5

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The synthesis of THF.

An early step in the industrial production of THF involves reaction of formaldehyde with acetylene to make 2-butyne-1,4-diol. This intermediate is hydrogenated and cyclised in two more steps to yield THF. The acetylene input is derived from fossil fuels, which again are non-renewable.

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The synthesis of 2-methyl THF.

An alternative to THF is 2-methyltetrahydrofuran, which has a very similar structure to THF.  It can be synthesized starting from biomass; after conversion to C5 and C6 sugars and subsequent acid-catalyzed steps, the intermediate levulinic acid can be hydrogenated to yield 2-methyl THF.

References:

  1. “Renewable Feedstocks for the Production of Chemicals” Bozell, J. J. ACS Fuels Preprints 1999, 44 (2), 204-209.
  2. “Conversion of Biomass into Chemicals over Metal Catalysts” Besson, M.; Gallezot, P.; Pinel, C. Chem. Rev. 2014, 114 (3), 1827-1870.
  3. “Transformation of Biomass into Commodity Chemicals Using Enzymes or Cells” Straathof, A. J. J. Chem. Rev., 2014, 114 (3), 1871-1908.
  4. “Tetrahydrofuran” Müller, H. in Ullmann’s Encyclopedia of Industrial Chemistry 2002, 36, 47-54.Wiley-VCH, Weinheim. doi:10.1002/14356007.a26_221
  5. “Synthesis of 2-Methyl Tetrahydrofuran from Various Lignocellulosic Feedstocks: Sustainability Assessment via LCA” Khoo, H. H.; Wong, L. L.; Tan, J.; Isoni, V.; Sharratt, P. Resour. Conserv. Recy. 2015, 95, 174.
Green Polymer Chemistry: Approaches, Challenges, Opportunity

Green Polymer Chemistry: Approaches, Challenges, Opportunity

By Hyungjun Cho, Member-at-large for the GCI

I was recently inspired by an episode of podcast by NPR’s Planet Money called Oil #4: How Oil Got Into Everything. It told the story of Leo Baekeland’s invention of Bakelite, which is the plastic that made many commodities affordable for the masses.

Plastic is made of polymers, and many of the common items we use are made from one or more of these polymers. Examples of these polymers are polystyrene, polymethylmethacrylate, and polyethylene and some examples of common items that contain these polymers are Styrofoam™, Plexiglas®, and plastic bags, respectively. Polymers are synthesized by forming bonds between many molecules of same structure, called monomers.

Conventionally, these monomers are produced from chemicals derived from oil, which is a non-renewable feedstock. Environmentally conscious scientists have been trying to make polymers in a more eco-friendly way. The biggest challenge lies in how we obtain monomers from renewable sources.

There are two main approaches to this challenge. The first approach is to produce currently used monomers, such as styrene, from a renewable source. A literature review by Hernandez et al.1 called this approach bioreplacement. The biggest progress in this

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Figure 1. Engineered metabolic pathway to produce styrene from glucose. (1)

approach has been made by engineering the metabolic pathways of bacteria cultures. McKenna et al. 3 have been able to feed glucose to engineered E. coli to produce styrene and release it in the culture medium they are incubating in. The E. coli flask cultures were able to produce styrene to reach concentrations of up to 260 mg/L1,3. Figure 1 shows the metabolic pathway from glucose to styrene.

While this method of producing monomers is promising, there are road blocks that are hindering progress. The biggest issue is toxicity of styrene to the E. coli, which limits the maximum concentration of styrene in the bacterial culture (E. coli can only tolerate up to 300 mg/L styrene1,3). Other challenges that exist with using bacteria include long incubation times, obtaining poor yield of desired product relative to amount of glucose added, and scale up. Looking down the road, these kinds of limitations may prevent this method from being economically and practically viable.

The second approach is called bioadvantage. Polymer chemists take chemicals that are already being produced from renewable feedstock, synthesize polymers, and use said polymers to produce polymer products in hopes of replacing already existing polymer materials. There are many molecules that are being studied for this purpose such as cellulose, starch, anethole, methylene-butyrolactone, and myrcene.

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Figure 2. Conventional monomers (styrene, methylmethacrylate, ethylene) and their potentially renewable counterparts. Renewable counterpart monomers tend to be structural analogues of conventional monomers.

During the podcast by Planet Money, research by the Hillmyer group from University of Minnesota was featured. They aim to synthesize eco-friendly polymer using monomers from renewable feedstock (the bioadvantage method). After many failures to produce viable polymer from corn, coconut, orange peels, etc., they were able to develop a polymer synthesized from a menthol derivative obtained from peppermint2.

A critical challenge to bioadvantage polymers is the need for years of study and passing a battery of regulatory tests before they are adopted. The petroleum based polymers that are being used today already have been researched for decades, which allows them to be used easily by industry. By extension, bioadvantage polymers will need to match or exceed their performance in terms of strength, durability, flexibility, and other properties we require from our plastic. Even when industry is willing to allocate resources to adopt eco-friendly polymers, sometimes it’s the consumers that prove to be even less accommodating. We observed this with the biodegradable bag fiasco by Sun Chips.

It should be mentioned that both bioreplacement and bioadvantage polymers are not necessarily biodegradable. Therefore, we should not call them green polymers.

I will conclude with this: I see the impact that plastic has on our daily lives and I see demand for polymers. Being able to make eco-friendly polymers economically will change the world around you, literally. As Planet Money teaches, the world works in a supply-demand swing. When the kinks in the supply side of eco-friendly polymers are fixed, demand for them will present itself. How soon eco-friendly plastics will develop will depend on us. As green chemists, we should see that the biggest impact we might have in the future, will be making eco-friendly polymers.

References

(1)   Hernández, N.; Williams, R. C.; Cochran, E. W. Org. Biomol. Chem., 2014,12, 2834-2849

(2)   Hillmyer, M. A.; Tolman, W. B. Acc. Chem. Res., 201447 (8), pp 2390–2396

(3)   Mckenna, R.; Nielsen, D. R. Metab. Eng. 2011, 13 (5), 544–554.

UofT Demonstrates its Commitment to Sustainable Chemistry

“We’re very pleased and proud to announce that the Chemistry Department has recently joined the Green Chemistry Commitment (GCC)!” – Dr. Andy Dicks, University of Toronto, Associate Professor

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GCI Members Fall 2016

The University of Toronto has recently signed the GCC making us the first school outside of the United States to sign onto this impactful commitment, which now contains 33 colleges and universities. The GCC is overseen by Beyond Benign, a United States not-for-profit organization created by Dr. Amy Cannon and Dr. John Warner, a founder of the principles of green chemistry. Within the GCC, academic institutions collaborate to share resources and know-how in order to positively impact how the next generation of scientists are educated about sustainability issues. Participating departments commit to green chemistry instruction as a core teaching mandate. The aim is to provide undergraduates and graduates with the required understanding to make green chemistry become standard practice in laboratories around the world. This, in turn, ensures that when graduates of the university enter the workforce, they are armed with the knowledge of how to make molecules and processes more sustainable and less toxic by adhering to the Twelve Principles of Green Chemistry.

The GCC unites the green chemistry community around shared goals and a common vision to grow departmental resources to allow a facile integration of green chemistry into the undergraduate laboratories as well as to improve connections with industry which creates job opportunities for sustainability-minded graduates. Their website offers many resources for those interested in reading actual case studies and laboratory exercises, so please click here to visit their website and be informed!

Our chemistry department has already improved the green chemistry content in our undergraduate laboratories by updating the first year courses and upper year synthetic chemistry courses to include various graded questions about the Twelve Principles as well as ensuring the undergraduates are thinking about how they could make their current lab protocols more sustainable. Additionally, students can choose to study the fate of chemicals in our environmental chemistry courses offered. Of course there’s always room to improve, so the Green Chemistry Initiative (GCI), in collaboration with Dr. Andy Dicks, is working on evaluating the undergraduate chemistry curriculum’s current focus on sustainable chemistry and toxicology, in hopes to further improve our undergraduate’s learning experience. The GCI also provides many educational opportunities to department members such as our Seminar Series as well as many outreach opportunities, making our group a driving force in the integration of green chemistry principles to the department. Lastly, the University of Toronto chemistry courses reach thousands of students a year, and by being the first Canadian university to sign this commitment, we are working towards a greener future in Canada!

Thank you for celebrating this very momentous achievement with us!
Karl Demmans, Ian Mallov, Shira Joudan, and Laura Reyes

Green Innovations – The Sky’s the Limit

By Annabelle Wong, 2016 Symposium Coordinator for the GCI

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Every year, 80 billion gallons of fuels are consumed and 705 million tones of CO2 is produced by airplanes. Innovations in chemistry for applications like futuristic windowless airplanes being developed by the Centre for Process Innovation is one way to reduce energy consumption and CO2 emissions. [1,2]

It’s an exciting time of year again when the GCI hosts their annual Green Chemistry Symposium! I remember starting as a graduate student here at the University of Toronto just a year ago and had the wonderful opportunity of attending the 2015 symposium as a first-timer. And this year, I will be attending the annual event as the GCI Symposium Coordinator.

What does this blog post have to do with windowless planes you may ask? The theme of the this year’s symposium is “Innovations in Chemistry towards Sustainable Urban Living” which will focus on topics related to greener products and chemical processes associated with urbanization and modern technological challenges like sustainable aerospace materials. The symposium organizing committee has chosen this theme because it builds onto last year’s theme of “Green Chemistry Applied to Industry” since innovations are often related to commercialization of products and scaling up in a cost-effective, sustainable manner.

With increasing interest coming from other departments at U of T and outside of U of T, we decided to expand this year’s symposium to include a public keynote lecture by Dr. John Warner, one of the founders of Green Chemistry, and an exciting case study session on analyzing a chemical process led by Dr. Tom Enright from Xerox Research Centre of Canada. We also decided to expand beyond just chemistry to touch on some chemical or process engineering topics.

The idea to include participants outside of chemistry partially stemmed from my personal experience working as an intern at the Fuel Cell Division of Mercedes Benz Canada in Vancouver and BASF SE in Germany. I realized that for chemists in academia, research often just stops at the chemical laboratory. But when it comes to research and development of a product in hopes of bringing it to the “real world” modern daily living, you’ll most likely find yourself interacting with an interdisciplinary team of scientists,  chemical, materials, mechanical, electrical or process engineers, and financial managers to ensure that the chemistry is cost-effective, safe, and sustainable to scale up. What might be seen as a novel innovative chemical reaction that works incredibly well in the laboratory scale may possibly end up as a disaster when it’s scaled up.  I think that the professional development in academia is slightly lacking when it comes to educating us on bridging the gap between chemistry and engineering and am delighted to have invited Dr. Enright from XRCC to teach us how to make this connection.

We are also very honored to have experts from academia and industry to tell us about their innovations in chemistry and how they can help the modern society to be a sustainable one. Topics include sustainability in textiles, electrochromic windows, catalysis, aerospace materials, switchable materials, biofuels, crop protection, and sustainable scale-up processes! Here’s the schedule of the symposium:gci blog 3

To find out how innovations in chemistry can make our world more sustainable or how your own research can take flight as a scalable innovation, make sure to register here before the deadline on May 2, 2016! See you there!

 

References:

[1] Centre for Process Innovation. Aerospace Windowless Aircraft – The Future Inspired by CPI. YouTube, https://www.youtube.com/watch?v=afgl5gx6avs (accessed April 27, 2016).

[2] 2014. The Centre for Process Innovation. http://www.uk-cpi.com/news/the-windowless-cabin-with-a-view/ (accessed April 27, 2016).